Delayed coking has been practiced for many years. The process utilizes thermal decomposition of heavy liquid hydrocarbons to produce coke, gas and liquid product streams of varying boiling ranges. The resulting coke is generally treated as a low value by-product, but is recovered for various uses, depending upon its quality.
The use of heavy crude oils having high metals and sulfur content is increasing in many refineries, and delayed coking operations are of increasing importance to refiners. The goal of minimizing air pollution is a further incentive for treating residuum in a delayed coking unit since the gases and liquids produced contain sulfur in a form that can be relatively easily removed.
Coking is a carbon rejection process in which low-value atmospheric or vacuum distillation bottoms are converted to lighter products which in turn can be hydrotreated to produce transportation fuels, such as gasoline and diesel. Coking of residuum from heavy high sulfur, or sour, crude oils is carried out primarily as a means of utilizing such low value hydrocarbon streams by converting part of the material to more valuable liquid and gas products.
In the commercial practice of the delayed coking process, the feedstock is first introduced into a fractionating column where lighter materials are recovered from the top and the bottoms are then sent to a coking furnace where they are rapidly heated to a coking temperature in the range of 480° to 530° C. and then fed to the coking drum. Coking units are typically configured with two parallel drums and operated in a swing mode. When one of the drums is filled with coke, the feed is transferred to the empty parallel drum. Liquid and gas streams from the coke drum are fed to the coking product fractionator.
Any hydrocarbon vapors remaining in the coke drum are removed by steam injection. The coke is cooled with water and then removed from the coke drum using hydraulic and/or mechanical means.
In the delayed coking production of fuel grade coke and, to some extent, even in the production of anode or aluminum grade coke, it is desirable to minimize the coke yield and maximize the liquid product yield, since the liquids are more valuable than the coke. It is also desirable to produce a coke having a volatile matter content of not more than about 15 percent by weight, and preferably in the range of 6 to 12 percent by weight.
In the conventional delayed coking process, fresh feedstock is introduced into the lower part of the coking fractionator for preheating and mixing and the fractionator bottoms, which include the heavy recycle material, and the fresh feedstock are heated to coking temperature in a coking furnace. The hot mixed fresh and recycle feedstream is introduced into a coke drum maintained at coking conditions of temperature and pressure where the feed decomposes or cracks to form coke and volatile components. The volatile components are recovered as vapor and transferred to the coking unit product fractionator. Heavy gas oil from the fractionator is added to the flash zone of the fractionator to condense the heaviest components from the coking unit product vapors. The heaviest fraction of the coke drum vapors can be condensed by other techniques, such as heat exchange, but in commercial operations it is common to contact the incoming vapors with heavy gas oil in the coking unit product fractionator. Conventional heavy recycle oil is comprised of condensed coking unit product vapors and unflashed heavy gas oil. When the coke drum is full of coke, the feed is switched to another drum, and the full drum is cooled and emptied by conventional methods as described above.
It is also known to add one or more catalysts and additives to the fresh feed and/or the fresh and recycle oil mixture prior to heating the feedstream in the coking unit furnace. The catalyst is used to promote the cracking of the heavy hydrocarbon compounds and the formation of the more valuable liquids that can be subjected to hydrotreating processes downstream to form transportation fuels. The catalyst and any additive(s) remain in the coking unit drum with the coke if they are solids or are present on a solid carrier; if the catalyst(s) and additive(s) are soluble in the oil, they are carried with the vapors and remain in the liquid products.
Processes have been disclosed for modifying the properties of the coke formed in the coking unit to obtain a particular coke product. For example, a delayed coking process is described in U.S. Pat. No. 4,713,168 in which Lewis acids, such as aluminum chloride, aluminum bromide, boron fluoride, zinc chloride and stannic chloride are used to obtain a premium coke having increased particle size. The additive and feedstock are introduced into the coking drum together. The additive can be in powder form or in liquid form if the feedstock is at a temperature above the melting point of the additive. The amount of the additive is a function of the feedstock used and the coking conditions employed. For example, 0.01 to about 5.0 percent by weight of additive based on the feedstock are used.
The use of additives based on polymeric materials with molecular weight in the range of from 1,000 to about 30,000 g/gmol is described in U.S. Pat. No. 7,658,838. The polymeric materials are selected from polyoxyethylene, polyoxypropylene, polyoxyethylene-polyoxypropylene copolymer, ethylene diamine tetra alkoxylated alcohol of polyoxyethylene alcohol, ethylene diamine tetra alkoxylated alcohol of polyxopropylene-polyoxyethylene alcohols and mixtures thereof and having a molecular weight from about 1,000 to about 30,000. The polymeric additive which is effective for the formation of substantially free-flowing shot coke is introduced into the feedstock at a point upstream of the second heating zone, between second heating zone and coking zone, or both.
A delayed coking process is described in U.S. Pat. No. 7,303,664 that utilizes metal complexes, where the metal is selected from the group consisting of vanadium, nickel, iron, tin, molybdenum, cobalt and sodium. The additives enhance the production of free-flowing shot coke during delayed coking. The feedstock is subjected to treatment with one or more additives at effective temperatures, i.e., from 70° C.-500° C. The additives can be in liquid or solid form. The additives include metal hydroxides, naphthenates and/or carboxylates, metal acetylacetonates, Lewis acids, metal sulfides, metal acetate, metal carbonates, high surface area metal-containing solids, inorganic oxides and salts of oxides, of which the basic salts are preferred additives.
A process is described in U.S. Pat. No. 7,645,375 in which low molecular weight hydrocarbons are used as additives to produce free-flowing shot coke. The feedstock is subjected to treatment with one or more additives at effective temperatures 70° C.-500 C. The additives include one- and two-ring aromatic systems having from about one to four alkyl substituents, which alkyl substituents contain about one to eight carbon atoms, preferably from about one to four carbon atoms. The one or more rings can be aromatic rings only or aromatic rings containing nitrogen, oxygen, sulfur. The additives, which include benzene, toluene, xylenes, methyl naphthalenes, dimethylnaphthates, indans, methyl indans, pyridine, methylpyridines, quinoline, and methylquinolines, are used in the concentration range of from 10 ppmw-30,000 ppmw.
A delayed coking process is described in U.S. Pat. No. 7,306,713 wherein metal free additives are used to produce free-flowing shot coke. The additives include elemental sulfur, high surface area substantially metal-free solids, such as rice hulls, sugars, cellulose, ground coals, ground auto tires; inorganic oxides such as fumed silica; salts of oxides, such as ammonium silicate and mineral acids such as sulfuric acid, phosphoric acid, and acid anhydrides.
An additive preparation method and utilization is described in U.S. Pat. Nos. 6,387,840, 6,193,875 and 6,169,054 for delayed coking process. The additives include metal salts containing a metal selected from the group consisting of alkali metals, alkaline earth metals, and mixtures thereof.
Gaseous hydrogen and hydrogen donor solvents are also utilized to enhance the coking unit product yields and quality. Hydrogen is used to stabilize the free radicals formed to increase liquid yields and, as a necessary result, to decrease the coke yield.
A delayed coking process is described in U.S. Pat. Nos. 4,698,147 and 4,178,229 in which a heavy hydrocarbon oil is admixed with a hydrogen donor diluent boiling in the range 200-540° C. The spent hydrogen donor is separated from the delayed coker products, regenerated and then recycled back to the coking unit.
U.S. Pat. No. 4,797,197 describes a delayed coking process wherein hydrogen gas is injected to stabilize a hydrocarbon compound incapable of further bimolecular reaction with another radical. This reaction is the reverse of coking reaction and hence minimizes coke production.
The references discussed above use additives/catalysts to improve the coke quality, but none of the references disclose a suitable, cost-effective additive, catalyst or adsorbent that can selectively remove the HPNA molecules from the liquid coking unit products to thereby enhance the quality of those products. A problem thus exists for producing transportation fuels from residual feedstocks that are low in HPNA molecules. A further problem exists when the feedstock contains metal compounds that remain in the coking unit product stream and are preferably removed or reduced prior to further processing of the various fractionator streams.